I needed string.Format in SQL just now, so I made it up (not very exciting but nicer than writing nested REPLACE statements. YMMV

 

DROP FUNCTION dbo.fnFormatString
GO
CREATE FUNCTION dbo.fnFormatString(@format varchar(6000),
    @arg1 sql_variant,
    @arg2 sql_variant='',
    @arg3 sql_variant='')
RETURNS varchar(8000)
AS
BEGIN
RETURN  REPLACE(
    REPLACE(
    REPLACE(@format, '{0}', CAST(@arg1 as varchar(400))),
    '{1}', CAST(@arg2 as varchar(400))),
    '{2}', CAST(@arg3 as varchar(400)))
END
GO
PRINT dbo.fnFormatString('This {0} of works like {1} in that langauge {2}'
    , 'sorta'
    , 'string.Format()'
    , 'C#')